On-press development of high speed laser sensitive lithographic printing plates

ABSTRACT

A method of on-press developing a laser sensitive lithographic printing plate with ink and/or fountain solution is described. The printing member comprises on a substrate a photosensitive layer soluble or dispersible in ink and/or fountain solution and capable of hardening upon exposure to a laser. The plate is exposed with a laser and on-press developed with ink and/or fountain solution. The exposure and development are performed with the plate under lightings that contain no or substantially no radiation below a wavelength selected from 400 to 650 nm, or in the dark or substantially dark.

FIELD OF THE INVENTION

This invention relates to lithographic printing plates. Moreparticularly, it relates to on-press ink and/or fountain solutiondevelopment of high-speed laser sensitive lithographic plates having ona substrate a photosensitive layer that has limited stability in ambientroom light.

BACKGROUND OF THE INVENTION

Lithographic printing plates (after process) generally consist ofink-receptive areas (image areas) and ink-repelling areas (non-imageareas). During printing operation, an ink is preferentially received inthe image areas, not in the non-image areas, and then transferred to thesurface of a material upon which the image is to be produced. Commonlythe ink is transferred to an intermediate material called printingblanket, which in turn transfers the ink to the surface of the materialupon which the image is to be produced.

At the present time, lithographic printing plates (processed) aregenerally prepared from lithographic printing plate precursors (alsocommonly called lithographic printing plates) comprising a substrate anda photosensitive coating deposited on the substrate, the substrate andthe photosensitive coating having opposite surface properties. Thephotosensitive coating is usually a photosensitive material, whichsolubilizes or hardens upon exposure to an actinic radiation, optionallywith further post-exposure overall treatment. In positive-workingsystems, the exposed areas become more soluble and can be developed toreveal the underneath substrate. In negative-working systems, theexposed areas become hardened and the non-exposed areas can be developedto reveal the underneath substrate. The exposed plate is usuallydeveloped with a liquid developer to bare the substrate in thenon-hardened or solubilized areas.

On-press developable lithographic printing plates have been disclosed inthe literature. Such plates can be directly mounted on press afterexposure to develop with ink and/or fountain solution during the initialprints and then to print out regular printed sheets. No separatedevelopment process before mounting on press is needed. Among thepatents describing on-press developable lithographic printing plates areU.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110,5,811,220, 6,014,929, 6,071,675, and 6,482,571).

Phase-switchable lithographic printing plates have also been disclosedin the literature which comprise on a substrate a photosensitive layer,wherein the photosensitive layer has the same surface affinity as thesubstrate (to ink or fountain solution), and is capable of switching tothe opposite affinity upon exposure with a radiation (with or withoutfurther treatment). The plate may be contacted with ink and fountainsolution on press to form inked images in the exposed (or non-exposed)areas and clean background in the non-exposed (or exposed) areas.

In one embodiment of such phase-switchable plates, the imaged areasswitch to opposite phase and also become hardened upon exposure to anactinic radiation. The non-exposed areas may be removed before or aftermounting on press. For the latter, the non-exposed areas may be removedwith ink and/or fountain solution during the initial or later stage ofprinting operation. Such plates are described in, for example, U.S. Pat.Nos. 6,331,375, 5,910,395, 6,740,464, and 6,136,503.

Conventionally, the plate is exposed with an actinic light (usually anultraviolet light from a lamp) through a separate photomask film havingpredetermined image pattern which is placed between the light source andthe plate. While capable of providing plate with superior lithographicquality, such a method is cumbersome and labor intensive.

Laser sources have been increasingly used to imagewise expose a printingplate which is sensitized to a corresponding laser wavelength. Thisallows the elimination of the photomask film, reducing material,equipment and labor cost. Suitable lasers include infrared lasers (suchas laser diode of about 830 nm and NdYAG laser of about 1064 nm),visible lasers (such as frequency-doubled NdYAG laser of about 532 nm,violet laser diode of about 390-430 nm), and ultraviolet laser (such asultraviolet laser diode of about 350 to 370 nm). Among them, infraredlaser diode, violet laser diode, and ultraviolet laser diode are mostattractive. Infrared laser sensitive plates have the advantage ofrelative white or yellow light stability, violet laser sensitive plateshave the advantage of low imager cost due to the low cost of the violetlaser diode which is made in mass production for DVD, and ultravioletlaser diode sensitive plates have the advantage of higher sensitivity(requiring less laser dosage) than longer wavelength lasers.

Laser sensitive plates generally have higher sensitivity (thanconventional film based plate) because of the limited laser power andthe desire for fast imaging speed. Accordingly, photosensitive platesdesigned for laser imaging generally have limited room light stability.For example, before being developed to remove the non-hardened areas,frequency-doubled NdYAG laser sensitive plates usually require red roomlight for handling, violet laser sensitive plates usually require orangeor yellow room light for handling, and infrared laser sensitivephotopolymer plates usually require yellow room light for handling andhave only limited white light stability (due to the use of certaininitiator which has spectral sensitivity in the ultraviolet region).

Such limited room light stability is an inherent barrier for the designand use of on-press developable laser sensitive lithographic platebecause the pressrooms are generally equipped with white lights, inaddition to the difficulties in designing any on-press developable platewith good press performance (including film based on-press developableplate). Despite of such difficulties, there is a strong desire todevelop a high speed laser sensitive on-press developable lithographicplate as well as methods of using it because of its environmental andeconomic benefits.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method of on-pressdeveloping a high speed laser sensitive lithographic printing platecomprising on a substrate a photosensitive layer which has limitedstability at ambient room light.

It is another object of this invention to provide a method of on-pressdeveloping a laser sensitive lithographic printing plate comprising on asubstrate a photosensitive layer which is on-press developable with inkand/or fountain solution.

It is yet another object of this invention to provide a method ofon-press imaging and developing a photosensitive lithographic platecomprising on a substrate a photosensitive layer which is on-pressdevelopable with ink and/or fountain solution.

Further objects, features and advantages of the present invention willbecome apparent from the detailed description of the preferredembodiments.

According to the present invention, there has been provided a method oflithographically printing images on a receiving medium, comprising inorder:

-   -   (a) providing a lithographic plate comprising (i) a substrate;        and (ii) a photosensitive layer capable of hardening upon        exposure to a laser, the non-hardened areas of said        photosensitive layer being soluble or dispersible in ink (for        waterless plate) or in ink and/or fountain solution (for wet        plate), and the hardened areas of said photosensitive layer        exhibiting an affinity or aversion substantially opposite to the        affinity or aversion of said substrate to at least one printing        liquid selected from the group consisting of ink and an abhesive        fluid for ink;    -   (b) imagewise exposing the plate with said laser to cause        hardening of the photosensitive layer in the exposed areas;    -   (c) contacting said exposed plate with ink and/or fountain        solution on a lithographic press to remove the photosensitive        layer in the non-hardened areas; and    -   (d) lithographically printing images from said plate to the        receiving medium;    -   (e) wherein the steps (b) and (c) are performed with the plate        under lightings that contain no or substantially no radiation        below a wavelength selected from 400 to 650 nm, or in the dark        or substantially dark.

Here the lightings (or darkness) for steps (b) and (c) can be differentor the same. The lighting can be from any light source, includingfiltered fluorescence light, filtered incandescence light, filteredsunlight, yellow light from any source, any red light from any source,and mixture of lights from different sources.

In one embodiment of this invention, the photosensitive layer exhibitsan affinity or aversion substantially opposite to the affinity oraversion of said substrate to at least one printing liquid selected fromthe group consisting of ink and an abhesive fluid for ink. Thephotosensitive layer does not switch its affinity or aversion upon laserexposure.

In another embodiment of this invention, the photosensitive layerexhibits substantially the same affinity or aversion as said substrateand is capable of switching to opposite affinity or aversion to saidsubstrate upon exposure to said laser (with or without furthertreatment).

The laser used in this invention can be any laser with a wavelengthselected from 200 to 1200 nm (including ultraviolet, visible, andinfrared lasers). The plate can be imagewise exposed with a laser on aplate exposure device and then transferred to a lithographic press foron-press development with ink and/or fountain solution by rotating theplate cylinder and engaging ink and/or fountain solution roller. Thedeveloped plate can then directly print images to the receiving sheets(such as paper). Alternatively, the plate can be imagewise exposed withthe laser on a lithographic press, on-press developed on the same presscylinder with ink and/or fountain solution, and then directly printimages to the receiving sheets.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The substrate employed in the lithographic plates of this invention canbe any lithographic support. Such a substrate may be a metal sheet, apolymer film, or a coated paper. Aluminum (including aluminum alloy)sheet is a preferred metal support. Particularly preferred is analuminum support that has been grained and anodized (with or withoutdeposition of a barrier layer). Polyester film is a preferred polymericfilm support. A surface coating may be coated to achieve desired surfaceproperties. For wet plate, the substrate should have a hydrophilic oroleophilic surface, depending on the surface properties of thephotosensitive layer; commonly, a wet lithographic plate has ahydrophilic substrate and an oleophilic photosensitive layer. Forwaterless plate, the substrate should have an oleophilic or oleophobicsurface, depending on the surface properties of the photosensitivelayer.

Particularly suitable hydrophilic substrate for a wet lithographic plateis an aluminum support that has been grained and anodized; such asubstrate is preferably further deposited with a hydrophilic barrierlayer. Surface graining (or roughening) can be achieved by mechanicalgraining or brushing, chemical etching, and/or AC electrochemicalgraining. The roughened surface can be further anodized to form adurable aluminum oxide surface using an acid electrolyte such assulfuric acid and/or phosphoric acid. The roughened and anodizedaluminum surface can be further thermally or electrochemically coatedwith a layer of silicate or hydrophilic polymer such as polyvinylphosphonic acid, polyacrylamide, polyacrylic acid, polybasic organicacid, copolymers of vinyl phosphonic acid and acrylamide to form adurable hydrophilic layer. Polyvinyl phosphonic acid and its copolymersare preferred polymers. Processes for coating a hydrophilic barrierlayer on aluminum in lithographic plate application are well known inthe art, and examples can be found in U.S. Pat. Nos. 2,714,066,4,153,461, 4,399,021, and 5,368,974. Suitable polymer film supports fora wet lithographic plate include a polymer film coated with ahydrophilic layer, preferably a hydrophilic layer that is crosslinked,as described in U.S. Pat. No. 5,922,502.

For preparing lithographic printing plates of the current invention, anyphotosensitive layer is suitable which is capable of hardening uponexposure to a laser having a wavelength selected from 200 to 1200 nm,and is soluble or dispersible in ink (for waterless plate) or in inkand/or fountain solution (for wet plate). Here hardening means becominginsoluble and non-dispersible in ink and/or fountain solution. Hardeningis generally achieved through crosslinking or polymerization of theresins (polymers or monomers). A laser sensitive dye or pigment isusually used in the photosensitive layer. The photosensitive layerpreferably has a coverage of from 100 to 4000 mg/m², and more preferablyfrom 400 to 2000 mg/m².

Photosensitive layer suitable for the current invention may beformulated from various photosensitive materials, usually with additionof a sensitizing dye or pigment. The composition ratios (such as monomerto polymer ratio) are usually different from conventional platesdesigned for development with a regular liquid developer. Variousadditives may be added to, for example, allow or enhance on-pressdevelopability. Such additives include surfactant, plasticizer, watersoluble polymer or small molecule, and ink soluble polymer or smallmolecule. The addition of nonionic surfactant is especially helpful inmaking the photosensitive layer dispersible with ink and fountainsolution, or emulsion of ink and fountain solution. Various additivesuseful for conventional photosensitive layer can also be used. Theseadditives include pigment, dye, exposure indicator, and stabilizer.

In this patent, the term monomer includes both monomer and oligomer, andthe term (meth)acrylate includes acrylate and/or methacrylate (acrylate,methacrylate, or both acrylate and methacrylate). In calculating theweight ratio of the monomer to the polymeric binder, the weight of themonomer includes the total weight of all monomers and the weight of thepolymeric binder includes the total weight of all polymeric binders. Theterm yellow or red light means yellow light, red light, or any lightwith a color between yellow and red such as orange light.

Photosensitive materials useful in wet plates of this invention include,for example, photosensitive compositions comprising a polymerizablemonomer, an initiator, a sensitizing dye, and optionally a polymer.

Photosensitive oleophobic materials useful in waterless plates of thisinvention include, for example, compositions comprising a monomer havingperfluoroalkyl or polysiloxane groups and crosslinkable terminal groups,an initiator, and a sensitizing dye.

Infrared laser sensitive materials useful for wet lithographic plates ofthis invention include, for example, thermosensitive compositionscomprising a polymerizable monomer, an initiator, an infrared lightabsorbing dye, and optionally a polymer.

Visible or ultraviolet light sensitive materials useful for wet platesof this invention include, for example, photosensitive compositionscomprising a polymerizable monomer, an initiator, a visible orultraviolet light sensitizing dye, and optionally a polymer.

Polymeric binder for the photosensitive layer of this invention can beany film-forming polymer. The polymers may or may not have(meth)acrylate groups or other polymerizable double bonds such as allylgroups. Examples of suitable polymers include (meth)acrylic polymers andcopolymers (such as polybutylmethacrylate, polyethylmethacrylate,polymethylmethacrylate, polymethylacrylate,butylmethacrylate/methylmethacrylate copolymer, andmethylmethacrylate/methylmethacrylic acid copolymer), polyvinyl acetate,polyvinyl butyrate, polyvinyl chloride, styrene/acrylonitrile copolymer,styrene/maleic anhydride copolymer and its partial ester,nitrocellulose, cellulose acetate butyrate, cellulose acetatepropionate, vinyl chloride/vinyl acetate copolymer,butadiene/acrylonitrile copolymer, and polyurethane binder.

Suitable free-radical polymerizable monomers (including oligomers)include, for example, multifunctional acrylate monomers or oligomers,such as (meth)acrylate esters of ethylene glycol, trimethylolpropane,pentaerythritol, ethoxylated ethylene glycol and ethoxylatedtrimethylolpropane, multifunctional urethanated (meth)acrylate, andepoxylated (meth)acrylate; and oligomeric amine diacrylates. Urethanated(meth)acrylate with at least six (meth)acrylate groups andnon-urethanated (meth)acrylate with at least four (meth)acrylate groupsare preferred monomers because of their fast curing speed. Combinationsof urethanated (meth)acrylate and non-urethanated (meth)acrylate areespecially useful. Various monomer combinations are described in U.S.patent application Ser. No. 10/720,882, the entire disclosure of whichis hereby incorporated by reference, and can be used for the preparationof the photosensitive layer of this invention.

Suitable free-radical initiators include, for example, the derivativesof acetophenone (such as 2,2-dimethoxy-2-phenylacetophenone, and2-methyl-1-[4-(methylthio)phenyl]-2-morpholino propan-1-one), oniumsalts such as diaryliodonium hexafluoroantimonate, diaryliodoniumhexafluorophosphate, diaryliodonium triflate,(4-(2-hydroxytetradecyl-oxy)phenyl)phenyliodonium hexafluoroantimonate,(4-octoxyphenyl)phenyliodonium hexafluoroantimonate,bis(4-t-butylphenyl)iodonium hexafluorophosphate, triarylsulfoniumhexafluorophosphate, triarylsulfonium p-toluenesulfonate,(3-phenylpropan-2-onyl) triaryl phosphonium hexafluoroantimonate andN-ethoxy(2-methyl)pyridinium hexafluorophosphate, and the onium salts asdescribed in U.S. Pat. Nos. 5,955,238, 6,037,098 and 5,629,354; boratesalts such as tetrabutylammonium triphenyl(n-butyl)borate,tetraethylammonium triphenyl(n-butyl)borate, diphenyliodoniumtetraphenylborate, and triphenylsulfonium triphenyl(n-butyl)borate, andthe borate salts as described in U.S. Pat. Nos. 6,232,038 and 6,218,076;haloalkyl substituted s-triazines such as2,4-bis(trichloromethyl)-6-(p-methoxy-styryl)-s-triazine,2,4-bis(trichloromethyl)-6-(4-methoxy-naphth-1-yl)-s-triazine,2,4-bis(trichloromethyl)-6-piperonyl-s-triazine, and2,4-bis(trichloromethyl)-6-[(4-ethoxyethylenoxy)-phen-1-yl]-s-triazine,and the s-triazines as described in U.S. Pat. Nos. 5,955,238, 6,037,098,6,010,824, and 5,629,354; titanocene such asbis(η⁹-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl)titanium; hexaarylbiimidazole compounds such as2,2′-bis(2-chlorophenyl)-4,4′,5,5′-tetraphenyl-1,1′-biimidazole,2,2′-bis(2-ethoxyphenyl)-4,4′,5,5′-tetraphenyl-1,1′-biimidazole,2-(1-naphthyl)-4,5-diphenyl-1,2′-biimidazole; and derivatives ofacetophenone such as 2,2-dimethoxy-2-phenylacetophenone, and2-methyl-1-[4-(methylthio)phenyl]-2-morpholino propan-1-one.Triarylsulfonium salts, diaryliodonium salts, and triarylalkylboratesalts are particularly suitable for infrared laser sensitive plate.Titanocene compounds and hexaarylbiimidazole compounds are particularlysuitable for visible or ultraviolet laser sensitive plate. The initiatoris added in the photosensitive layer preferably at 0.1 to 40% by weightof the photosensitive layer, more preferably 1 to 30%, and mostpreferably 5 to 20%.

Suitable polyfunctional epoxy monomers include, for example,3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate,bis-(3,4-epoxycyclohexymethyl) adipate, difunctional bisphenolA/epichlorohydrin epoxy resin and multifunctionalepichlorohydrin/tetraphenylol ethane epoxy resin.

Suitable cationic photoinitiators include, for example, triarylsulfoniumhexafluoroantimonate, triarylsulfonium hexafluorophosphate,diaryliodonium hexafluoroantimonate, and haloalkyl substituteds-triazine. It is noted that most cationic initiators are also freeradical initiators because, in addition to generating Bronsted acid,they also generate free radicals during photo or thermal decomposition.

Infrared sensitizing dyes useful in the infrared sensitivephotosensitive layer (also called thermosensitive layer) of thisinvention include any infrared absorbing dye effectively absorbing aninfrared radiation having a wavelength of 750 to 1200 nm. It ispreferable that the dye has an absorption maximum between thewavelengths of 800 and 1100 nm. Various infrared absorbing dyes aredescribed in U.S. Pat. Nos. 5,858,604, 5,922,502, 6,022,668, 5,705,309,6,017,677, and 5,677,106, and in the book entitled “Infrared AbsorbingDyes” edited by Masaru Matsuoka, Plenum Press, New York (1990), and canbe used in the thermosensitive layer of this invention. Examples ofuseful infrared absorbing dyes include squarylium, croconate, cyanine(including polymethine), phthalocyanine (including naphthalocyanine),merocyanine, chalcogenopyryloarylidene, oxyindolizine, quinoid,indolizine, pyrylium and metal dithiolene dyes. Cyanine andphthalocyanine dyes are preferred infrared absorbing dyes.

Visible or ultraviolet sensitizing dyes useful in the visible orultraviolet sensitive photosensitive layer of this invention include anydyes having a wavelength maximum of from 200 to 600 nm and capable ofdirectly or indirectly causing polymerization of the monomers uponexposure to the corresponding laser. Usually, the visible or ultravioletdye activates an initiator to cause the polymerization of the monomerupon exposure to a laser. Suitable visible and ultraviolet sensitivedyes include, for example, cyanine dyes (including polymethine dyes);rhodamine compounds such as rhodamine 6G perchloride; chromanonecompounds such as 4-diethylaminobenzilidene chromanone;dialkylaminobenzene compounds such as ethyl 4-dimethylaminobenzoate anddialkylaminobenzene; dialkylaminobenzophenone compounds such as4,4′-bis(dimethylamino)benzophenone, 4,4′-bis(diethylamino)benzophenone,2-(p-dimethylaminophenyl)benzooxazole,2-(p-diethylaminophenyl)benzooxazole,2-(p-dimethylaminophenyl)benzo[4,5]benzooxazole,2-(p-dimethylaminophenyl)benzo[6,7]benzooxazole,2,5-bis(p-diethylaminophenyl) 1,3,4-oxazole,2-(p-dimethylaminophenyl)benzothiazole,2-(p-diethylaminophenyl)benzothiazole,2-(p-dimethylaminophenyl)benzimidazole,2-(p-diethylaminophenyl)benzimidazole,2,5-bis(p-diethylaminophenyl)1,3,4-thiadiazole,(p-dimethylaminophenyl)pyridine, (p-diethylaminophenyl)pyridine,2-(p-dimethylaminophenyl)quinoline, 2-(p-diethylaminophenyl)quinoline,2-(p-dimethylaminophenyl)pyrimidine or2-(p-diethylaminophenyl)pyrimidine; unsaturated cyclopentanone compoundssuch as2,5-bis{[4-(diethylamino)phenyl]methylene}-(2E,5E)-(9Cl)-cyclopentanoneand bis(methylindolenyl)cyclopentanone; coumarin compounds such as3-benzoyl-7-methoxy coumarin and 7-methoxy coumarin; and thioxanthenecompounds such as 2-isopropylthioxanthenone. Dialkylaminobenzenecompounds and bis(dialkylamino)benzophenone compounds are particularlysuitable for ultraviolet laser sensitive plate.Bis(dialkylamino)benzophenone compounds are particularly suitable forviolet laser sensitive plate. The sensitizing dyes as described in U.S.Pat. Nos. 5,422,204 and 6,689,537, and U.S. Pat. App. Pub. No.2003/0186165 can be used for the photosensitive layer of this invention.

The photosensitive composition of the present invention preferablycontains a hydrogen-donor compound as a polymerization accelerator.Examples of the hydrogen-donor compound include compounds having amercapto group such as 2-mercaptobenzothiazole, 2-mercaptobenzimidazole,2-mercaptobenzoxazole and 3-mercapto-1,2,4-triazole, N,N-dialkyl benzoicalkyl ester, N-aryl-α-amino acids, their salts and esters such asN-phenylglycine, salts of N-phenylglycine, and alkyl esters ofN-phenylglycine such as N-phenylglycine ethyl ester and N-phenylglycinebenzyl ester.

Various surfactants may be added into the photosensitive layer to allowor enhance the on-press developability with ink and/or fountain. Bothpolymeric and small molecule surfactants can be used. However, it ispreferred that the surfactant has low or no volatility so that it willnot evaporate from the photosensitive layer of the plate during storageand handling. Nonionic surfactants are preferred. The nonionicsurfactant used in this invention should have sufficient portion ofhydrophilic segments (or groups) and sufficient portion of oleophilicsegments (or groups), so that it is at least partially soluble in water(>1 g surfactant soluble in 100 g water) and at least partially solublein organic phase (>1 g surfactant soluble in 100 g photosensitivelayer). Preferred nonionic surfactants are polymers and oligomerscontaining one or more polyether (such as polyethylene glycol,polypropylene glycol, and copolymer of ethylene glycol and propyleneglycol) segments. Examples of preferred nonionic surfactants are blockcopolymers of propylene glycol and ethylene glycol (also called blockcopolymer of propylene oxide and ethylene oxide); ethoxylated orpropoxylated acrylate oligomers; and polyethoxylated alkylphenols andpolyethoxylated fatty alcohols. The nonionic surfactant is preferablyadded at from 0.1 to 30% by weight of the photosensitive layer, morepreferably from 0.5 to 20%, and most preferably from 1 to 15%.

For plates with rough and/or porous surface capable of mechanicalinterlocking with a coating deposited thereon, a thin water solubleinterlayer may be deposited between the substrate and the photosensitivelayer. Here the substrate surface is rough and/or porous enough and theinterlayer is thin enough to allow bonding between the photosensitivelayer and the substrate through mechanical interlocking. Such a plateconfiguration is described in U.S. Pat. No. 6,014,929, the entiredisclosure of which is hereby incorporated by reference. Preferredreleasable interlayer comprises a water-soluble polymer. Polyvinylalcohol (including various water-soluble derivatives of polyvinylalcohol) is the preferred water-soluble polymer. Usually purewater-soluble polymer is coated. However, one or more surfactant andother additives may be added. The water-soluble polymer is generallycoated from an aqueous solution with water as the only solvent. Awater-soluble organic solvent, preferably an alcohol such as ethanol orisopropanol, can be added into the water-soluble polymer aqueous coatingsolution to improve the coatability. The water-soluble organic solventis preferably added at less than 20% by weight of the solution, morepreferably at less than 10%. The releasable interlayer preferably has anaverage coverage of 1 to 200 mg/m², more preferably 2 to 100 mg/m², andmost preferably 4 to 40 mg/m². The substrate preferably has an averagesurface roughness Ra of 0.2 to 2.0 microns, and more preferably 0.4 to1.0 microns.

The photosensitive layer may be conformally coated onto a roughenedsubstrate (for example, with Ra of larger than 0.4 microns) at thincoverage (for example, of less than 1.2 g/m²) so that the plate can havemicroscopic peaks and valleys on the photosensitive layer coated surfaceand exhibit low tackiness and good block resistance, as described inU.S. Pat. No. 6,242,156, the entire disclosure of which is herebyincorporated by reference.

A water soluble or dispersible overcoat can be coated on thephotosensitive layer to, for example, improve the photospeed, surfacedurability, and/or on-press developability of the plate. The overcoatpreferably comprises a water-soluble polymer, such as polyvinyl alcohol(including various water-soluble derivatives of polyvinyl alcohol).Combination of two or more water-soluble polymers (such as a combinationof polyvinyl alcohol and polyvinylpyrrolidone) may also be used.Polyvinyl alcohol is a preferred water-soluble polymer. Variousadditives, such as surfactant, wetting agent, defoamer, leveling agentand dispersing agent, can be added into the overcoat formulation tofacilitate, for example, the coating or development process. Examples ofsurfactants useful in the overcoat of this invention includepolyethylene glycol, polypropylene glycol, and copolymer of ethyleneglycol and propylene glycol, polysiloxane surfactants, perfluorocarbonsurfactants, alkylphenyl ethylene oxide condensate, sodiumdioctylsulfosuccinate, sodium dodecylbenzenesulfonate, and ammoniumlaurylsulfate. Various organic or inorganic emulsion or dispersion maybe added into the overcoat to, for example, reduce the tackiness ormoisture sensitivity of the plate. The overcoat preferably has acoverage of from 0.001 to 3.0 g/m², more preferably from 0.005 to 1.0g/m², and most preferably from 0.01 to 0.15 g/m².

In a preferred embodiment for the thermosensitive lithographic printingplates of this invention, the thermosensitive layer comprises at leastone polymeric binder (with or without ethylenic functionality), at leastone polymerizable ethylenically unsaturated monomer having at least oneterminal ethylenic group, at least one free-radical initiator capable ofgenerating free radical at elevated temperature and/or through chargetransfer from a radiation-activated infrared dye, and at least oneinfrared absorbing dye or pigment. Other additives such as surfactant,dye or pigment, exposure-indicating dye (such as leuco crystal violet,leucomalachite green, azobenzene, 4-phenylazodiphenylamine, andmethylene blue dyes), and free-radical stabilizer (such asmethoxyhydroquinone) may be added. The monomer-to-polymer weight ratiois preferably larger than 1, more preferably larger than 1.5, and mostpreferably larger than 2.0.

In another preferred embodiment for the thermosensitive lithographicprinting plates of this invention, the thermosensitive layer comprises apolymeric binder, a urethane (meth)acrylate monomer having at least 6(meth)acrylate groups, a non-urethane (meth)acrylate monomer having atleast 4 (meth)acrylate groups, a free-radical initiator, and an infraredabsorbing dye. The weight ratio of the urethane (meth)acrylate monomerto the non-urethane (meth)acrylate monomer is preferably from 0.10 to3.0, more preferably from 0.15 to 2.0, even more preferably from 0.20 to1.5, and most preferably from 0.30 to 1.0.

In a preferred embodiment for visible or ultraviolet light sensitivelithographic printing plates of this invention, the photosensitive layercomprises at least one polymeric binder (with or without ethylenicfunctionality), at least one polymerizable ethylenically unsaturatedmonomer having at least one terminal ethylenic group, at least onefree-radical initiator, and at least one visible or ultravioletsensitizing dye. Other additives such as surfactant, dye or pigment,exposure-indicating dye, and free-radical stabilizer may be added. Themonomer-to-polymer weight ratio is preferably larger than 1, morepreferably larger than 1.5, and most preferably larger than 2.0.

In another preferred embodiment for visible or ultraviolet lightsensitive lithographic printing plates of this invention, thephotosensitive layer comprises a polymeric binder, a urethane(meth)acrylate monomer having at least 6 (meth)acrylate groups, anon-urethane (meth)acrylate monomer having at least 4 (meth)acrylategroups, a free-radical initiator, and a visible or ultravioletsensitizing dye. The weight ratio of the urethane (meth)acrylate monomerto the non-urethane (meth)acrylate monomer is preferably from 0.10 to3.0, more preferably from 0.15 to 2.0, even more preferably from 0.20 to1.5, and most preferably from 0.30 to 1.0.

On-press developable lithographic plates as described in U.S. Pat. Nos.6,482,571, 6,576,401, 5,548,222, and 6,541,183, the entire disclosuresof which are hereby incorporated by reference, can be used for theinstant invention.

A hydrophilic or oleophilic micro particles may be added into thephotosensitive layer to enhance, for example, the developability andnon-tackiness of the plate. Suitable micro particles include polymerparticles, talc, titanium dioxide, barium sulfate, silicone oxide, andaluminum micro particles, with an average particle size of less than 10microns, preferably less than 5 microns, more preferably less than 2microns, and most preferably less than 1 microns. A suitable particulardispersion is described in U.S. Pat. No. 6,071,675, the entiredisclosure of which is hereby incorporated by reference.

The hardened areas of the photosensitive layer should exhibit anaffinity or aversion substantially opposite to the affinity or aversionof the substrate to at least one printing liquid selected from the groupconsisting of ink and an abhesive fluid for ink. For example, a wetplate can have a hydrophilic substrate and an oleophilic photosensitivelayer, or can have an oleophilic substrate and a hydrophilicphotosensitive layer; a waterless plate can have an oleophilic substrateand an oleophobic photosensitive layer, or can have an oleophobicsubstrate and an oleophilic photosensitive layer. An abhesive fluid forink is a fluid that repels ink. Fountain solution is the most commonlyused abhesive fluid for ink. A wet plate is printed on a wet pressequipped with both ink and fountain solution, while a waterless plate isprinted on a waterless press equipped with ink.

Usually, as for most printing plates described in the literature, thephotosensitive layer exhibits an affinity or aversion substantiallyopposite to the affinity or aversion of the substrate to at least oneprinting liquid selected from the group consisting of ink and anabhesive fluid for ink, and does not switch its affinity or aversionupon laser exposure. However, certain photosensitive layer exhibitssubstantially the same affinity or aversion as the substrate and iscapable of switching to opposite affinity or aversion upon exposure to alaser (with or without further treatment), as described in U.S. Pat.Nos. 6,331,375, 5,910,395, 6,720,464, and 6,136,503. Bothnon-phase-switchable photosensitive layer and phase-switchablephotosensitive layer can be used for the current invention.Non-phase-switchable photosensitive layer is preferred.

Infrared lasers useful for the imagewise exposure of the thermosensitiveplates of this invention include laser sources emitting in the nearinfrared region, i.e. emitting in the wavelength range of from 750 to1200 nm, and preferably from 800 to 1100 nm. Particularly preferredinfrared laser sources are laser diodes emitting around 830 nm or aNdYAG laser emitting around 1060 nm. The plate is exposed at a laserdosage that is sufficient to cause hardening in the exposed areas butnot high enough to cause substantial thermal ablation. The exposuredosage is preferably from 1 to 400 mJ/cm², more preferably from 5 to 200mJ/cm², and most preferably from 20 to 100 mJ/cm², depending on thesensitivity of the thermosensitive layer.

Visible lasers useful for the imagewise exposure of the visible lightsensitive plates of this invention include any laser emitting in thewavelength range of from 390 to 600 nm. Examples of suitable visiblelasers include frequency-doubled Nd/YAG laser (about 532 nm), argon ionlaser (about 488 nm), violet diode laser (about 410 nm), and visibleLEDs. Violet laser diode is especially useful because of its small sizeand relatively lower cost. The exposure dosage is preferably from 0.0001to 5 mJ/cm² (0.1 to 5000 μJ/cm²), more preferably from 0.001 to 0.5mJ/cm² (1 to about 500 μJ/cm²), and most preferably from 0.005 to 0.10mJ/cm² (5 to 100 μJ/cm²), depending on the sensitivity of thephotosensitive layer.

Ultraviolet lasers useful for the imagewise exposure of the ultravioletlight sensitive plates of this invention include any laser having awavelength of from 200 to 390 nm. Examples of ultraviolet lasers includeultraviolet diode lasers or LEDs having a wavelength of from 350 to 390nm. Laser diodes are preferred ultraviolet lasers. The exposure dosageis preferably from 0.0001 to 5 mJ/cm² (0.1 to 5000 μJ/cm²), morepreferably from 0.001 to 0.5 mJ/cm² (1 to about 500 μj/cm²), and mostpreferably from 0.005 to 0.10 mJ/cm² (5 to 100 μJ/cm²), depending on thesensitivity of the photosensitive layer.

Laser imaging devices are currently widely available commercially. Anydevice can be used which provides imagewise laser exposure according todigital imaging information. Commonly used imaging devices includeflatbed imager, internal drum imager, and external drum imager, all ofwhich can be used for the imagewise laser exposure in this invention.

The on-press developable plate is usually exposed on an exposure device,and then mounted on press to develop with ink and/or fountain solutionand then print out regular printed sheets. However, the plate can alsobe exposed on a printing press (such as by mounting on the platecylinder or sliding through a flatbed imager mounted on the press), andthe exposed plate can be directly developed on press with ink and/orfountain solution and then print out regular printed sheets. The inkand/or fountain solution solubilized or dispersed photosensitive layerand/or overcoat can be mixed into the ink and/or the fountain solutionon the rollers, and/or can be transferred to the blanket and then thereceiving medium (such as paper). The fountain solution roller isengaged (to the plate cylinder as for conventional inking system or tothe ink roller as for integrated inking system) for preferably 0 to 100rotations, more preferably 1 to 50 rotations and most preferably 5 to 20rotations (of the plate cylinder), and the ink roller is then engaged tothe plate cylinder for preferably 0 to 100 rotations, more preferably 1to 50 rotations and most preferably 5 to 20 rotations before engagingthe plate cylinder and feeding the receiving medium (such as paper).Good quality prints should be obtained preferably under 40 initialimpressions, more preferably under 20 impressions, and most preferablyunder 5 impressions.

For conventional wet press, usually fountain solution is applied (tocontact the plate) first, followed by contacting with ink roller. Forpress with integrated inking/dampening system, the ink and fountainsolution are emulsified by various press rollers before beingtransferred to the plate as emulsion of ink and fountain solution.However, in this invention, the ink and fountain solution may be appliedat any combination or sequence, as needed for the plate. There is noparticular limitation. The recently introduced single fluid ink that canbe used for printing wet lithographic plate without the use of fountainsolution, as described in for example U.S. Pat. No. 6,140,392, can alsobe used for the on-press development and printing of the plate of thisinvention.

The plate may be rinsed or applied with an aqueous solution, includingwater and fountain solution, to remove the water soluble or dispersibleovercoat (for plate with an overcoat) and/or to dampen withoutdeveloping the plate, after imagewise exposure and before on-pressdevelopment with ink and/or fountain solution.

For lithographic plates having a free radical crosslinkablephotosensitive layer, a liquid layer may be applied onto the surface ofthe plate (with or without an overcoat) before and/or during imagingprocess to provide an in situ oxygen barrier layer during the imagingprocess to allow faster photospeed and better curing. The liquid layercan be any liquid material that is compatible with the plate. Water,fountain solution, and other aqueous solutions are preferred materialsfor forming the liquid layer. The liquid layer may be applied from adampening roller of a lithographic press with the plate being mounted onthe plate cylinder during on-press imaging process. The dampening rollercan be a regular dampening roller which supplies fountain solutionduring printing or can be a different roller.

For plates having a free radical polymerizable photosensitive layer, aninert gas (such as nitrogen) may be introduced within the device or nearthe exposure areas during a laser imaging process to reduce inhibitionof free radical polymerization of the photosensitive layer by oxygen.The inert gas may be flushed from a nozzle mounted next to the laserhead onto the areas being imaged during the laser imaging process; thisis especially useful for external drum imaging devices, includingoff-press laser imaging devices having an external drum and on-presslaser imaging devices utilizing plate cylinder as the imaging drum.

The ink used in this application can be any ink suitable forlithographic printing. Most commonly used lithographic inks include “oilbased ink” which crosslinks upon exposure to the oxygen in the air and“rubber based ink” which does not crosslink upon exposure to the air.Specialty inks include, for example, radiation-curable ink and thermallycurable ink. An ink is an oleophilic, liquid or viscous material whichgenerally comprises a pigment dispersed in a vehicle, such as vegetableoils, animal oils, mineral oils, and synthetic resins. Variousadditives, such as plasticizer, surfactant, drier, drying retarder,crosslinker, and solvent may be added to achieve certain desiredperformance. The compositions of typical lithographic inks are describedin “The Manual of Lithography” by Vicary, Charles Scribner's Sons, NewYork, and Chapter 8 of “The Radiation Curing: Science and Technology” byPappas, Plenum Press, New York, 1992.

The fountain solution used in this application can be any fountainsolution used in lithographic printing. Fountain solution is used in thewet lithographic printing press to dampen the hydrophilic areas(non-image areas), repelling ink (which is hydrophobic) from theseareas. Fountain solution contains mainly water, generally with additionof certain additives such as gum arabic and surfactant. Small amount ofalcohol such as isopropanol can also be added in the fountain solution.Water is the simplest type of fountain solution. Fountain solution isusually neutral to mildly acidic. However, for certain plates, mildlybasic fountain solution is used. The type of fountain solution useddepends on the type of the plate substrate as well as the plate. Variousfountain solution compositions are described in U.S. Pat. Nos. 4,030,417and 4,764,213.

Emulsion of ink and fountain solution is an emulsion formed from ink andfountain solution during wet lithographic printing process. Becausefountain solution (containing primarily water) and ink are not miscible,they do not form stable emulsion. However, emulsion of ink and fountainsolution can form during shearing, compressing, and decompressingactions by the rollers and cylinders, especially the ink rollers andplate cylinder, on a wet lithographic press. For wet press withintegrated inking system, ink and fountain solution are emulsified onthe ink rollers before transferred to the plate.

The laser exposure and the on-press development are independentlyperformed with the plate under a lighting that contains no orsubstantially no radiation below a wavelength selected from 400 to 650nm, or in the dark or substantially dark; preferably under a lightingthat contains no radiation below a wavelength selected from 400 to 650nm, or in the dark. Such lighting is usually a yellow or red light. Thisincludes a light that is from a fluorescence or incandescence lamp thatis covered with a filter that cuts off all or substantially all (atleast 99%) of the radiation below a wavelength selected from 400 to 650nm; preferably the lamp is covered with a filter that cuts off all ofthe radiation below a wavelength selected from 400 to 650 nm. The laserexposure and the on-press development can be performed with the plateunder the same or different lightings.

The term “substantially no radiation below a wavelength” means theintensity of the radiation below that wavelength is less than 1% of thatfor a regular 100-watt incandescence light (for home use, not focused)at a distance of 2 meters. The term “substantially dark” means theintensity of the radiation is less than 1% of that for a 100-wattincandescence light at a distance of 2 meters. For the purpose of thisapplication, both the office fluorescence light and incandescence lightare considered as white light.

The lighting that contains no or substantially no radiation below awavelength selected from 400 to 650 nm (usually as a yellow or redlight) allows a working lighting condition for the operators.Alternatively, the plate can be handled automatically in the dark,including in a dark room or in a light-tight box of any shape.Preferably, during the handling before exposure and the handling beforemounting on press, the plate is in a lighting or lightings that containno or substantially no radiation below a wavelength selected from 400 to650 nm, or in the dark or substantially dark.

In one embodiment of the instant invention, both the laser exposure andon-press development are performed under a lighting that contains no orsubstantially no radiation below a wavelength selected from 400 to 650nm; the lightings for the laser exposure and on-press development can bethe same or different. In the second embodiment, the laser exposure isperformed under a lighting that contains no or substantially noradiation below a wavelength selected from 400 to 650 nm and theon-press development is performed in the dark or substantially dark. Inthe third embodiment, the laser exposure is performed in the dark orsubstantially dark and the on-press development is performed under alighting that contains no or substantially no radiation below awavelength selected from 400 to 650 nm. In the fourth embodiment, boththe laser exposure and the on-press development are performed in thedark or substantially dark.

The plates can be packaged in a light-tight (or yellow or red lightpassing only) cassette. The cassette can be connected to the laserexposure device with light-tight (or yellow or red light passing only)covers for the plate being exposed. The plate can be automaticallytransferred to the exposure device for laser exposure. The exposed platecan be transferred to the press for on-press development manually underyellow or red light, or automatically under yellow or red light or inthe dark.

The plate on the press can be open to the room lighting which is ayellow or red light or in the dark or substantially dark. Preferably,the press is designed in a way so that the plate is fully covered with acover (or covers) which allows only yellow or red light passing through(allowing viewing) or no light passing through; this allows whitelighting for the pressroom.

For on-press exposure and development, the plate can be exposed on pressbefore mounting on the plate cylinder (such as exposed with a flatbedlaser scanner installed on the press), or can be exposed on the platecylinder. The plate on the press can be open to the room lighting whichis a yellow or red light or in the dark or substantially dark.Preferably, the press is designed in a way so that the plate is fullycovered with a cover (or covers) that cuts off all of the radiationbelow a wavelength selected from 400 to 650 nm, during exposure and/oron-press development. The plate can be loaded manually under a yellow orred room lighting. Preferably, the plate is automatically loaded ontothe press for on-press exposure and development from a light-tightcassette, under white room lighting.

The lighting used in this invention can be any light that contains no orsubstantially no radiation below a wavelength selected from 400 to 650nm. Such a cut off wavelength can be 400, 450, 500, 550, 600, and 650nm, or any wavelength between 400 and 650 nm, depending on the spectralsensitivity of the plate. Usually, such light is achieved by adding afiltering cover or coating to a white fluorescence or incandescence lampto cut off the radiation at shorter wavelength. Such light includesyellow and red lights (including any light with color between yellow andred, such as orange light). Various yellow and red lights arecommercially available (such as from EncapSulite International Inc. andGeneral Electric), and can be used for the instant invention.

This invention is further illustrated by the following examples of itspractice. Unless specified, all the values are by weight.

EXAMPLES 1-5

An electrochemically roughened, anodized, and polyvinylphosphonic acidtreated aluminum sheet was first coated with a 0.1% aqueous solution ofpolyvinyl alcohol (Celvol 540, from Celanese) with a #6 Meyer rod,followed by drying in an oven at 100° C. for 2 min. The polyvinylalcohol coated substrate was further coated with the photosensitivelayer formulation PS-1 with a #8 Meyer rod, followed by drying in anoven at 90° C. for 2 min. PS-1 Component Weight ratios Neocryl B-728(Polymer from Zeneca) 3.193 Sartomer SR-399 (Acrylic monomer fromSartomer) 7.630 Pluronic L43 (Nonionic surfactant from BASF) 0.6492,2-Bis(2-chlorophenyl)-4,4′,5,5′- 1.407 tetraphenyl-1,1′-biimidazole2-Mercaptobenzoxazole 0.839 4,4′-Bis(diethylamino)benzophenone 0.2812-Butanone 86.000

The photosensitive layer coated plate was further coated with awater-soluble overcoat OC-1 using a #6 Meyer rod, followed by drying inan oven at 100° C. for 2 min. OC-1 Component Weight ratios Airvol 205(polyvinyl alcohol from Air Products) 0.40 Dioctyl sulfosuccinate sodiumsalt (surfactant) 0.02 Water 99.58

The plate was exposed with a violet plate imager equipped with a 30 mwviolet laser diode emitting at about 405 nm (MAKO-4 from ECRM) for adosage of about 60 μJ/cm². The plate was imaged in a dim red light room,and was kept in a light tight box before and after imaging.

The laser exposed plate was cut into five pieces, and each piece waswrapped with a separate aluminum foil. Each piece of the plate wastested on press under a different lighting condition. The first piecewas tested in the dark (with all lights turned off). The second piecewas tested under a 60-watt yellow light (yellow coated incandescencelight, from General Electric). The third piece was tested under a60-watt red light (red coated incandescence light, from GeneralElectric). The fourth piece was tested under a 20-watt white officefluorescence light (from General Electric). The fifth piece was testedunder a 100-watt regular incandescence light (for home use, from GeneralElectric). Each light was about 2 meters from the top of the press. Ittook about 5 minutes to mount each plate and start up the press.

Each of the exposed plate pieces was unwrapped and tested on a wetlithographic press (AB Dick 360) under the above described lightingcondition (including dark). The plate was directly mounted on the platecylinder of the press. After starting the press, the fountain roller wasengaged for 20 rotations, the ink roller (carrying emulsion of ink andfountain solution) was applied to the plate cylinder for 20 rotations,and the plate cylinder was then engaged with the blanket cylinder andprinted with paper. The printed sheets were evaluated for the on-pressdevelopability of the plates, with the results summarized in Table 1.TABLE 1 Background Inking in Background at at 200 imaging Press roomlighting 20 impressions impressions areas In the dark (no light) CleanClean Good Yellow light Clean Clean Good Red light Clean Clean GoodWhite fluorescence light Inked Heavy toning Good Regular incandescenceInked Heavy toning Good light

EXAMPLES 6-10

An electrochemically roughened, anodized, and silicate treated aluminumsheet was coated with thermosensitive layer formulation PS-2 using a #8Meyer rod, followed by drying in an oven at 90° C. for 2 min. PS-2Component Weight ratios Neocryl B-728 (Polymer from Zeneca) 2.73Sartomer SR-399 (Acrylic monomer from Sartomer) 6.52 Pluronic L43(Nonionic surfactant from BASF) 0.562,4-Bis(trichloromethyl)-6-[(4-ethoxyethylenoxy)- 1.00phen-1-yl]-s-triazine ADS-830AT (Infrared absorbing cyanine dye from0.10 American Dye Source) Acetone 90.0

The thermosensitive layer coated plate was further coated with awater-soluble overcoat OC-2 using a #6 Meyer rod, followed by drying inan oven at 100° C. for 2 min. OC-2 Component Weight ratios Airvol 205(Polyvinyl alcohol from Air Products) 5.00 Zonyl FSO (Perfluorinatedsurfactant from DuPont) 0.02 Water 95.00

The plate was exposed with an infrared laser plate imager equipped withlaser diodes emitting at about 830 nm (Trendsetter from Creo) at adosage of 150 mJ/cm². The plate was imaged in a dim red light room, andwas kept in a light tight box before and after imaging.

The laser exposed plate was cut into five pieces, and kept in alight-tight box for all the time except for the specific exposure asindicated. The first piece was kept in the dark (in a box) all the timeafter exposure. The second piece was exposed to a 60-watt yellow light(yellow coated incandescence light, from General Electric) at a distanceof 2 meters for 60 minutes. The third piece was exposed to a 60-watt redlight (red coated incandescence light, from General Electric) at adistance of 2 meters for 60 minutes. The fourth piece was exposed to a40-watt white office fluorescence light (from General Electric) at adistance of 2 meters for 60 minutes. The fifth piece was exposed to a100-watt regular incandescence light (for home use, from GeneralElectric) at a distance of 2 meters for 60 minutes.

The exposed plate pieces as treated above were tested on a wetlithographic press (AB Dick 360) under a dim red light. The plate wasdirectly mounted on the plate cylinder of the press. After starting thepress, the fountain roller was engaged for 20 rotations, the ink roller(carrying emulsion of ink and fountain solution) was applied to theplate cylinder for 20 rotations, and the plate cylinder was then engagedwith the blanket cylinder and printed with paper. The printed sheetswere evaluated for the on-press developability of the plates, with theresults summarized in Table 2. TABLE 2 Room light Background Inking inexposure before Background at at 200 imaging mounting on press 20impressions impressions areas In the dark (no exposure) Clean Clean GoodYellow light for 60 Clean Clean Good minutes Red light for 60 minutesClean Clean Good White office fluorescence Inked Heavy toning Good lightfor 60 mintues Regular incandescence Inked Heavy toning Good light for60 minutes

1. A method of lithographically printing images on a receiving medium,comprising in order: (a) providing a lithographic plate comprising (i) asubstrate, and (ii) a photosensitive layer capable of hardening uponexposure to a laser having a wavelength selected from 200 to 1200 nm;said photosensitive layer being soluble or dispersible in ink (forwaterless plate) or in ink and/or fountain solution (for wet plate), andexhibiting an affinity or aversion substantially opposite to theaffinity or aversion of said substrate to at least one printing liquidselected from the group consisting of ink and an abhesive fluid for ink;(b) imagewise exposing the plate with said laser to cause hardening ofthe photosensitive layer in the exposed areas; (c) contacting saidexposed plate with ink and/or fountain solution on a lithographic pressto remove the photosensitive layer in the non-hardened areas; and (d)lithographically printing images from said plate to the receivingmedium; (e) wherein the steps (b) and (c) are performed with the plateunder lightings that contain no or substantially no radiation below awavelength selected from 400 to 650 nm, or in the dark or substantiallydark; the lightings (or darkness) for steps (b) and (c) being differentor the same.
 2. The method of claim 1 wherein said steps (b) and (c) areperformed with the plate in the dark (including in a light-tight box ofany shape).
 3. The method of claim 1 wherein said steps (b) and (c) areperformed with the plate under a lighting or lightings that do notcontain any radiation below a wavelength selected from 400 to 650 nm. 4.The method of claim 1 wherein said step (b) is performed with the plateunder a lighting that does not contain any radiation below a wavelengthselected from 400 to 650 nm, and said step (c) is performed with theplate in the dark.
 5. The method of claim 1 wherein said step (b) isperformed with the plate in the dark, and said step (c) is performedwith the plate under a lighting that does not contain any radiationbelow a wavelength selected from 400 to 650 nm.
 6. The method of claim 1wherein said plate is under a lighting or lightings that contain no orsubstantially no radiation below a wavelength selected from 400 to 650nm and/or in the dark or substantially dark all the time before thecompletion of step (c).
 7. The method of claim 1 wherein said plate isunder a lighting or lightings that contain no radiation below awavelength of 450 nm and/or in the dark all the time before thecompletion of step (c).
 8. The method of claim 1 wherein said plate isunder a lighting or lightings that contain no radiation below awavelength of 550 nm and/or in the dark all the time before thecompletion of step (c).
 9. The method of claim 1 wherein said steps (b)and (c) are performed with the plate under lightings that are fromfluorescence or incandescence lamps which are covered with a filter thatcuts off all the radiation below a wavelength selected from 400 to 650nm, the lightings for steps (b) and (c) being different or the same. 10.The method of claim 1 wherein said plate is exposed with said laser onsaid lithographic press.
 11. The method of claim 1 wherein said plate isexposed with said laser while mounted on the plate cylinder of saidlithographic press.
 12. The method of claim 1 wherein said plate isexposed with said laser through a flatbed imager mounted on saidlithographic press.
 13. The method of claim 1 wherein said laser is anultraviolet laser having a wavelength of about 350 to 390 nm, said plateis imagewise exposed with said laser at a dosage of less than 100μJ/cm², and said lighting contains no radiation below a wavelengthselected from 400 to 550 nm.
 14. The method of claim 1 wherein saidlaser is a violet laser having a wavelength of about 390 to 430 nm, saidplate is imagewise exposed with said laser at a dosage of less than 200μJ/cm², and said lighting contains no radiation below a wavelengthselected from 450 to 600 nm.
 15. The method of claim 1 wherein saidlaser is an infrared laser having a wavelength of about 800 to 1100 nm,said photosensitive layer is capable of hardening upon exposure withsaid infrared laser as well as with an ultraviolet radiation, and saidlighting contains no radiation below a wavelength selected from 400 to550 nm.
 16. The lithographic plate of claim 1 further including areleasable interlayer interposed between the substrate and thephotosensitive layer, said releasable interlayer being soluble ordispersible in ink and/or fountain solution; wherein the substratecomprises rough and/or porous surface capable of mechanical interlockingwith a coating deposited thereon, and the interlayer is substantiallyconformally coated on the microscopic surfaces of the substrate and isthin enough in thickness, to allow bonding between the photosensitivelayer and the substrate through mechanical interlocking.
 17. The methodof claim 1 wherein said plate further comprises a water soluble ordispersible overcoat.
 18. The method of claim 1 wherein saidphotosensitive layer comprises a free radical polymerizableethylenically unsaturated monomer, a free radical initiator, and asensitizing dye.
 19. A method of lithographically printing images on areceiving medium, comprising in order: (a) providing a lithographicplate comprising (i) a hydrophilic substrate; (ii) an oleophilicphotosensitive layer comprising a polymeric binder, a free radicalpolymerizable ethylenically unsaturated monomer, a free radicalinitiator, and a sensitizing dye; and (iii) a water soluble ordispersible overcoat; said photosensitive layer being soluble ordispersible in ink and/or fountain solution and capable of hardeningupon exposure to a laser having a wavelength selected from 300 to 430nm; (b) imagewise exposing the plate with said laser at a dosage of lessthan 200 μJ/cm² to cause hardening of the photosensitive layer in theexposed areas; (c) contacting said exposed plate with ink and/orfountain solution on a lithographic press to remove the photosensitivelayer in the non-hardened areas; and (d) lithographically printingimages from said plate to the receiving medium; (e) wherein the steps(b) and (c) are performed with the plate under lightings that contain noradiation below a wavelength selected from 400 to 650 nm, or in thedark; the lightings for steps (b) and (c) (if not in the dark) beingdifferent or the same.
 20. A method of lithographically printing imageson a receiving medium, comprising in order: (a) providing a lithographicplate comprising (i) a substrate, and (ii) a photosensitive layercapable of hardening upon exposure to a laser having a wavelengthselected from 200 to 1200 nm; said photosensitive layer being soluble ordispersible in ink (for waterless plate) or in ink and/or fountainsolution (for wet plate), and exhibiting substantially the same affinityor aversion as said substrate and being capable of switching to oppositeaffinity or aversion to said substrate upon exposure to said laser; (b)imagewise exposing the plate with the laser to switch the affinity oraversion of the photosensitive layer and cause hardening of thephotosensitive layer in the exposed areas; (c) contacting said exposedplate with ink and/or fountain solution on a lithographic press toremove the photosensitive layer in the non-hardened areas; and (d)lithographically printing images from said plate to the receivingmedium; (e) wherein the steps (b) and (c) are performed with the plateunder lightings that contain no or substantially no radiation below awavelength selected from 400 to 650 nm, or in the dark or substantiallydark; the lightings (or darkness) for steps (b) and (c) being differentor the same.